The present invention relates to a method for recording optically information by projecting a light beam modulated in accordance with the information to be recorded on a light sensitive record medium as a single spiral track or a number of concentrical tracks by means of an objective lens and to an apparatus for carrying out such an optical information recording method.
In such optical information recording method and apparatus, the light beam modulated in accordance with the information should be made incident upon the record medium along the precisely predetermined spiral track or concentric tracks. For the sake of simplicity these tracks are termed as circular tracks hereinafter. To this end, a so-called tracking control should be effected in accordance with a tracking error signal which represents an amount and a direction of deviation of the light beam with respect to the predetermined circular track; i.e. a tracking error. There have been proposed various methods for deriving the tracking error signal. In one method, the tracking error signal is obtained by
projecting two light beams onto a record medium by means of an objective lens;
modulating one of said two light beams (main beam) in accordance with an information signal;
focussing the other light beam (sub beam) onto an edge of an adjacent track which has been recorded by said main beam or onto an edge of a tracking track which has been previously formed for tracking; and
detecting a change in an amount of said sub beam reflected by the record medium by means of a light detector.
In the known apparatus for recording optically information mentioned above, the record medium is usually called as a video disc in which encoded video and audio signals are recorded as optical information such as optical transmitting, reflection and phase properties. One of important properties of such a record medium is a very high density of recorded information and thus, a width of the information track is very narrow and a space between successive tracks is also very narrow. In order to record the information in the video disc in the form of circular information tracks having very narrow width and pitch correctly, it is necessary not only to minimize a radius of a light spot on the disc plane all the time by driving the objective lens always in an in-focussed condition with respect to the plane of the video disc, but also to make a distance between successively recorded tracks constant exact.
In the usual information recording apparatus mentioned above, there might occur a problem that a direct current or very low frequency component of the tracking error signal is varied due to a variation of a light intensity of a light source, an irregularity of reflection factor of the disc and so on, and thus the recorded information tracks have a discrepancy from the correct spiral or concentrical tracks due to the variation of the direct current or low frequency component in the tracking error signal.